Neutron absorber consisting of refractory metal infused with discrete neutron absorber

ABSTRACT

The present invention provides a gray rod control assembly for a nuclear reactor. The gray rod control assembly includes a spider assembly having a plurality of radial extending flukes and a plurality of gray rod assemblies coupled to the flukes of the spider assembly. Each of the gray rod assemblies includes an elongated tubular member, a first end plug, a second end plug, and a neutron absorber. The neutron absorber includes a matrix of refractory metal fabricated to be porous into which a metal or metal alloy is infused. The neutron absorber is distributed among a plurality of the gray rod assemblies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to nuclear reactors and, moreparticularly, to an improved neutron absorber material contained in grayrod assemblies of gray rod control assemblies (GRCAs).

2. Description of the Prior Art

The fuel assemblies of modern reactor cores typically employ two typesof rod control assemblies to control reactivity, rod cluster controlassemblies (RCCAs) and gray rod control assemblies (GRCAs). Both consistof a plurality of neutron-absorbing rods fastened at their top ends to acommon hub or spider assembly. The body of the rods generally comprisesa stainless steel tube which encapsulates a neutron-absorbing material,such as a pure silver absorber material or a silver-indium-cadmium alloyabsorber material, and the rods are slid within tubular guide thimbletubes of the fuel assembly with a control drive mechanism near the topof the spider assembly operating to control the movement of the rodswithin the thimble tubes. In this manner, the controlled insertion andextraction of the rods generally controls the rate of reactor powerproduced.

The power produced by the reactor of a nuclear power plant is generallycontrolled by raising or lowering control rod assemblies within thereactor core, and the change in reactor power output required in orderto accommodate a change in the demand for electrical output from theelectrical power plant is commonly referred to as load follow. Asdescribed, for example, in U.S. Pat. No. 4,079,236, load follow presentsmany operating issues. For instance, in a pressurized water reactor(PWR) during load follow, reactivity must be controlled and axial powerdistribution changes in the core in response to the power level change,must be addressed.

Typically, GRCAs are used in load follow maneuvering because theycomprise reduced worth control rods, commonly referred to in the art as“gray” rods. Gray rods are known to provide a mechanical shim (MSHIM)reactivity mechanism as opposed to a chemical shim, which requireschanging the concentration of soluble boron in the reactor coolant.Thus, the use of gray rods minimizes the need for processing the primaryreactor coolant on a daily basis and, therefore, greatly simplifiesoperations. More specifically, GRCA designs typically consist oftwenty-four rodlets fastened at their top ends to the spider. Of thetwenty-four rodlets within the cluster, only four rods are absorberrods, and the neutron-absorber material encapsulated within the absorberrods typically consists of about 80% silver, about 15% indium, and about5% cadmium. Such a design poses several disadvantages.

Among the disadvantages of known GRCA designs, is the fact that indiumand cadmium have relatively large neutron cross-sections, which resultin their depletion over a relatively short period of time. Silverdepletes somewhat more slowly than indium and cadmium, and ultimatelytransmutes into other non-absorbing isotopes of cadmium. As a result ofcontinued decrease in the rod worth, the GRCAs become less effective incontrolling the reactivity of the reactor during load follow. Inaddition, elemental transmutation of silver and indium to other metalsleads to changes in absorber material properties and excessive absorberswelling, which has been a recognized problem in the industry for manyyears. This undesirably leads to frequent GRCA replacement.

A second disadvantage relates to changes in the local rod power for fuelrods which are adjacent to the four guide thimbles that contain theabsorber rods. Specifically, because the absorber material is localizedto four rodlets, a rapid change in power, commonly referred to as thedelta-power of the fuel rods, occurs, for example, during a rod pull. Arod pull is the process of extracting the GRCA from the fuel assembly,and in GRCA designs it results in a delta-power spike.

There exists a need, therefore, for an improved neutron absorbermaterial for gray rod assemblies which overcomes the aforementioneddisadvantages typically found in known GRCAs.

SUMMARY OF THE INVENTION

This need and others are satisfied by the present invention, which isdirected to an improved neutron absorbing material for gray rod controlassemblies (GRCAs), which is mechanically self-supporting up tosubstantially higher temperatures than those at which pure silver orsilver-indium-cadmium alloy retains its shape, while being spatiallyuniform in its ability to absorb neutrons.

In one aspect of the present invention, there is provided a gray rodassembly for a gray rod control assembly of a nuclear reactor. Thenuclear reactor includes a number of fuel assemblies each having aplurality of elongated nuclear fuel rods supported in an organized arrayby a number of substantially transverse support grids, and a pluralityof guide thimbles disposed through the support grids and along the fuelrods. The gray rod control assembly includes a spider assembly having aplurality of radially extending flukes and being structured to move eachgray rod assembly within one of the guide thimbles in order to controlthe rate of power produced by the nuclear reactor. The gray rod assemblycomprises an elongated tubular member having a first end, a second end,an inner diameter, and a length; a first end plug coupled to the firstend of the elongated tubular member, and being structured to facilitateinsertion of the elongated tubular member into one of the guide thimblesof the fuel assembly; a second end plug coupled to the second end of theelongated tubular member, and being structured to be coupled to one ofthe radially extending flukes of the spider assembly of the gray rodcontrol assembly; and a neutron absorber comprised of a matrix ofrefractory metal fabricated to be porous into which a neutron absorbingmetal or metal alloy is infused. The neutron absorber is disposed as aplurality of segments within most of the elongated tubular member,having a diameter which is relatively equivalent in diameter to theelongated tubular member, and a length which is shorter than the lengthof the elongated tubular member, in order to minimize the exposedsurface area of the neutron absorber to radiation when the tubularmember is inserted into the thimble and to allow the tubular member toflex, if necessary. The neutron absorber is distributed among aplurality of the gray rod assemblies.

In another aspect of the present invention, there is provided a gray rodcontrol assembly for a nuclear reactor. The nuclear reactor includes aplurality of fuel assemblies each having a plurality of elongatednuclear fuel rods supported in an organized array by a number ofsubstantially transverse support grids, and a plurality of guidethimbles disposed through the support grids and along the fuel rods. Thegray rod control assembly comprises a spider assembly having a pluralityof radially extending flukes; and a plurality of gray rod assembliescoupled to the flukes of the spider assembly, the spider assembly beingstructured to move each gray rod assembly within one of the guidethimbles in order to control the rate of power produced by the nuclearreactor. Each of the gray rod assemblies comprises an elongated tubularmember having a first end, a second end, an inner diameter, and alength; a first end plug coupled to the first end of the elongatedtubular member, and being structured to facilitate insertion of theelongated tubular member into one of the guide thimbles of the fuelassembly; a second end plug coupled to the second end of the elongatedtubular member, and being structured to be coupled to one of theradially extending flukes of the spider assembly; and a neutronabsorber.

The neutron absorber may comprise a matrix of refractory metalfabricated to be porous into which a neutron absorbing metal or metalalloy is infused. The neutron absorber is disposed as a plurality ofsegments within the elongated tubular member generally toward the firstend. The neutron absorber has a diameter that is relatively equivalentin diameter to the elongated tubular member, and a length that issubstantially shorter than the length of the elongated tubular member.The neutron absorber is distributed among a plurality of the gray rodassemblies.

In a further aspect of the present invention, there is provided anuclear reactor, comprising a plurality of elongated nuclear fuel rodseach having an extended axial length; a number of substantiallytransverse support grids spaced along the axial length of the fuel rodsin order to hold the fuel rods in an organized array; a plurality ofguide thimbles disposed through the support grids and along the fuelrods; and a gray rod control assembly including a spider assembly havinga plurality of radially extending flukes, and a plurality of gray rodsassemblies coupled to the flukes, the advanced gray rod control assemblybeing structured to move each of the gray rod assemblies within one ofthe guide thimbles in order to control the rate of power produced by thenuclear reactor. Each of the gray rod assemblies comprises an elongatedtubular member having a first end, a second end, an inner diameter, anda length, a first end plug coupled to the first end of the elongatedtubular member, the first end plug being tapered in order to facilitateinsertion of the elongated tubular member into one of the guide thimblesof the fuel assembly, a second end plug coupled at one end to the secondend of the elongated tubular member, and at the other end to one of theradially extending flukes of the spider assembly, and a neutronabsorber.

The neutron absorber may comprise a matrix of refractory metalfabricated to be porous into which a neutron absorbing metal or metalalloy is infused. The neutron absorber is disposed as a plurality ofsegments generally filling most of the elongated tubular member. Theneutron absorber has a diameter that is relatively equivalent to theinner diameter of the elongated tubular member. The neutron absorber isdistributed among a plurality of the gray rod assemblies.

The neutron absorber is comprised preferably of between about 40% toabout 80% refractory metal and between about 20% to about 60% metal ormetal alloy, more preferably of between about 50% to about 70%refractory metal and between about 30% to about 50% metal or metalalloy, and most preferably of about 65% refractory metal and about 35%metal or metal alloy.

Suitable refractory metals of the present invention include, withoutlimitation, molybdenum, tungsten, niobium or zirconium.

An exemplary neutron absorber of the present invention includes, forexample, a matrix of porous molybdenum as a refractory metal that isinfused with silver or a silver-indium-cadmium alloy in the pore networkof the refractory metal. The porous matrix of the refractory metal isaccomplished, for example, by sintering.

The neutron absorber material of the present invention may be shaped,for example and without limitation, as cylindrical pellets such as rightcircular cylindrical pellets.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the invention can be gained from the followingdescription of the preferred embodiments when read in conjunction withthe accompanying drawings in which:

FIG. 1 is an elevational view of a fuel assembly, illustrated invertically shortened form, and a control assembly therefore, partiallyshown in hidden line drawing;

FIG. 2A is a partially sectioned elevational view of the controlassembly of FIG. 1, which has been removed from the fuel assembly;

FIG. 2B is a top plan view of the control rod spider assembly for thecontrol assembly of FIG. 2A;

FIG. 3 is a partially sectioned elevational view of a gray rod assemblynot to scale in accordance with the invention; and

FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For simplicity of disclosure, the invention will be described withreference to the pressurized water reactor (PWR) core design which iscommercially known under the designation AP1000. The AP1000 is aWestinghouse Electric Company LLC reactor design. Westinghouse ElectricCompany LLC has a place of business in Monroeville, Pa. Reference to theAP1000 is provided for illustrative example purposes only and is notlimiting upon the scope of the invention. It will, therefore, beappreciated that the exemplary GRCA design of the invention hasapplication in a wide variety of other reactor designs.

Directional phrases used herein, such as, for example, upper, lower,top, bottom, left, right, and derivatives thereof, relate to theorientation of the elements shown in the drawings and are not limitingupon the claims unless expressly recited therein.

As employed herein, the statement that two or more parts are “coupled”together shall mean that the parts are joined together either directlyor joined through one or more intermediate parts.

As employed herein, the term “number” shall refer to one and more thanone (i.e., a plurality).

Fuel Assembly

Referring now to the drawings and particularly to FIG. 1, there is shownan elevational view of a nuclear reactor fuel assembly, represented invertically shortened form and being generally designated by referencenumeral 10. The fuel assembly 10 is the type used in a pressurized waterreactor (PWR) and has a structural skeleton which, at its lower end,includes a bottom nozzle 12 for supporting the fuel assembly 10 on alower core support plate 14 in the core region of the nuclear reactor(not shown), a top nozzle 16 at its upper end, and a number of guidetubes or thimbles 18 which extend longitudinally between and are rigidlyattached at opposite ends, to the bottom and top nozzles 12 and 16.

The fuel assembly 10 further includes a plurality of transverse grids 20axially-spaced along and mounted to the guide thimble tubes 18 and, anorganized array of elongated fuel rods 22 transversely-spaced andsupported by the grids 20. The assembly 10 also has an instrumentationtube 24 located in the center thereof and extending between and mountedto the bottom and top nozzles 12 and 16. In view of the foregoingarrangement of parts, it will be understood that the fuel assembly 10forms an integral unit capable of being conveniently handled withoutdamaging the assembly parts.

As previously discussed, the array of fuel rods 22 in the fuel assembly10 are held in spaced relationship with one another by the grids 20which are spaced along the fuel assembly length. Each fuel rod 22includes nuclear fuel pellets 26 and is closed at its opposite ends byupper and lower end plugs 28 and 30. The pellets 26 are maintained in astack by a plenum spring 32 disposed between the upper end plug 28 andthe top of the pellet stack. The fuel pellets 26, composed of fissilematerial, are responsible for creating the reactive power of thereactor. A liquid moderator/coolant such as water or water containingboron, is pumped upwardly through a plurality of flow openings in thelower core plate 14 to the fuel assembly. The bottom nozzle 12 of thefuel assembly 10 passes the coolant upwardly through the guide thimbles18 and along the fuel rods 22 of the assembly in order to extract heatgenerated therein for the production of useful work. To control thefission process, a number of control rods 33 without neutron absorberand control rods 34 with neutron absorber are reciprocally moveable inthe guide thimbles 18 located at predetermined positions in the fuelassembly 10. A spider assembly 39 positioned above the top nozzle 16supports the control rods 33, 34.

FIGS. 2A and 2B show the control rod assembly 36 after it has beenremoved from the fuel assembly 10 of FIG. 1. Generally, the controlassembly 36 has an internally threaded cylindrical member 37 with aplurality of radially-extending flukes or arms 38, which comprise thespider assembly 39, best shown in FIG. 2B. As previously discussed, eacharm 38 is interconnected to the control rods 33 without neutron absorberand control rods 34 with neutron absorber, such that the control rodassembly 36 is operable to move the control rods 33, 34 verticallywithin the guide thimbles 18 (FIG. 1) to thereby control the fissionprocess in the fuel assembly 10 (FIG. 1), all in a well known manner.With the exception of the exemplary control rod assembly which comprisesa gray control rod assembly (GRCA) 36 having gray rod assemblies 34 withimproved neutron absorbing material, which will now be discussed, all ofthe foregoing is old and generally well known in the art.

Improved GRCA

Continuing to refer to FIGS. 2A and 2B, the general control rodconfiguration will now be described. As previously stated, in order totake advantage of the MSHIM capabilities afforded by low worth or grayrods, known control rod assemblies, such as the existing controlassemblies for the Westinghouse Electric Company LLC AP1000 reactor,employ GRCAs. However, while the GRCA design for the current AP1000reactor design has twenty-four rods which are generally configured asshown in FIG. 2B, as mentioned hereinbefore, twenty of the twenty-fourrods are stainless steel (e.g., without limitation, SS-304) waterdisplacing rods and only four of the rods are neutron-absorber rods.Therefore, essentially all of the neutron absorber material is localizedand isolated in only four rod locations within the GRCA.

Additionally, in the existing AP1000 design, the absorber materialcomprises an Ag—In—Cd absorber consisting of about 80% silver, about 15%indium, and about 5% cadmium. This absorber material is consistent withknown standard full-strength rod cluster control assemblies (RCCAs), inwhich all twenty-four rods are Ag—In—Cd. As noted, indium and cadmiumare known to rapidly deplete. RCCAs spend a minimal amount of time inthe core during power operation. Therefore, such depletion is not anissue. However, for the AP1000 mechanical shim (MSHIM) operation, forexample, the GRCAs are expected to reside in the core for up to half ofthe operating cycle. Under these operating conditions, the existing GRCAdesign would need to be replaced periodically due to rapid absorberdepletion. As will be discussed in detail herein, among other benefits,the improved GRCA design of the invention overcomes this rapid depletiondisadvantage and also substantially avoids the undesirable local powerspike experienced when traditional GRCA having four gray rod assemblieswith neutron absorbing material is pulled from the core.

Two-dimensional multi-group transport theory simulations alsodemonstrate that the exemplary neutron absorber of the invention,comprised of, for example and without limitation, a molybdenumrefractory metal infused with silver, compared to gray rod designscomposed of thin wires of pure silver metal or Ag—In—Cd surrounded by asteel spacer sleeve and a clad with approximately equal reactivityworth, is superior both in terms of depletion lifetime andintra-assembly peaking. Depletion calculations indicate thatapproximately one-third of the silver in the exemplary silver-molybdenumneutron absorber of the invention transmute to cadmium-108 or -110 atthe end of its targeted lifetime. The molybdenum percentage, however,remains essentially constant over the targeted life of the gray rodassembly 34 (FIGS. 2A, 2B), with no significant quantities of otherchemical species produced due to irradiation of the molybdenum. Hence,the material composition of the irradiated silver-molybdenum neutronabsorber of the invention is expected to remain fairly similar to theinitial composition. In contrast, depletion calculations for smalldiameter applications of Ag—In—Cd alloy indicate that the relativechanges in material composition due to irradiation are significantlylarger in these materials for the same targeted lifetime.

A further understanding of the aforementioned absorber depletion issuewill be had with reference to FIGS. 3 and 4, which show the gray rodassembly 34 with neutron absorber 110 of the invention. As shown in FIG.3, the gray rod assembly 34 generally includes a first end 40 which, asoriented in the core (FIG. 1), is the bottom end, and a second end 42(e.g., top end from the perspective of FIG. 1). The first or bottom end40 has a tapered end plug 44. Such tapered design facilitates guidedinsertion of the rod 34 into the thimble tube 18 (FIG. 1) of the fuelassembly 10 (FIG. 1). The second or top end 42 has a top end plug 46which is structured to engage and secure to the spider assembly 39 (bestshown in FIG. 2A) in a known manner (e.g., without limitation, acomplementary male/female fastening arrangement). An elongated tubularmember 48 extends between the top and bottom end plugs 46, 44. Theexemplary tubular member 48 is a stainless steel tube made from304-stainless steel, although tubes made from other known or suitablealternative materials are contemplated. In the example shown anddiscussed herein, the inner diameter 50 of the tube 48 (FIG. 4) is about0.38 inches (0.97 centimeters). However, it will be appreciated that theconcepts of the invention are equally applicable for gray rod assemblies34 having any suitable inner diameter for use in a wide variety ofreactors.

The neutron absorber material 110 comprises a matrix of porousrefractory metal infused with a neutron absorbing metal or metal alloy.The refractory metal is fabricated, for example, by compacting andsintering metal powder so as to result in a continuous pore network,which then is infused with a neutron absorbing metal or metal alloy. Theneutron absorber 110 preferably is between about 40% to about 80%refractory metal and between about 20% to about 60% neutron absorbingmetal or metal alloy, more preferably between about 50% to about 70%refractory metal and between about 30% to about 50% neutron absorbingmetal or metal alloy, and most preferably about 65% refractory metal andabout 35% neutron absorbing metal or metal alloy. The refractory metalmay be, for example, molybdenum, tungsten, niobium or zirconium. Asdiscussed above, preferably, the refractory metal is molybdenum and themetal that is infused in the refractory metal preferably is, forexample, silver. Alternatively, a neutron absorbing metal alloy such as,for example, Ag—In—Cd, may be infused in the refractory metal.

The neutron absorber material 110 generally is disposed within most ofthe tube 48 in a plurality of segments 58 therein, shown not in scale inFIG. 3, in which each segment 58 comprises the neutron absorber 110,preferably in the form, for example, of cylindrical pellets.Segmentation of the neutron absorber 110 in the elongated tubular member48 allows for the elongated tubular member 48 to be flexible so as toreduce frictional forces between the elongated tubular member 48 and thethimble tubes 18, which reduces the likelihood of incomplete insertionof the gray rod assemblies 34 in the thimble tubes 18.

As best shown in the cross-sectional view of FIG. 4, the diameter 54 ofthe exemplary neutron absorber material 110 is relatively equivalent indiameter to the inner diameter 50 of the rod tube 48. The length 56 ofthe absorber 110 in the example of FIG. 3 is about 166 inches (421.64centimeters). Although, as with the other dimensions of the gray rod 34,this measurement could vary without departing from the scope of theinvention.

The refractory metal of the invention serves as a structural componentfor the neutron absorbing material infused therein, i.e., the silvermetal or silver-indium-cadmium metal alloy is infused in the refractorymetal in relatively small amounts in the pore network of the refractorymetal.

The exemplary gray rod assembly 34 of the invention provides an extendednuclear lifetime through use of the exemplary neutron absorber 110. Thisis due to the low percentage of absorber metal, for example, about 35%silver, infused in the porous matrix of refractory metal, for example,about 65% molybdenum. Specifically, the low percentage of absorber metalresists bulk boiling during conditions of high local power density,absorber swelling and resultant clad cracking. The overall GRCA 36design of the invention also generally improves fuel rod 22 linear heatrate change margins during GRCA 36 maneuvers.

In an embodiment, of the twenty-four rods 33, 34 in the exemplary GRCA36, about, for example, twelve rods 34 contain the exemplary neutronabsorber 110 and the remaining rods 33 do not contain the exemplaryneutron absorber 110, as opposed to localizing the absorber in only fourrods, as in the existing AP1000 design discussed hereinbefore. However,it will be appreciated that the concepts of the invention are equallyapplicable for gray rods 33, 34 having any other suitable arrangement ofneutron absorber-containing rods 34 and rods 33 without neutronabsorber.

In addition, the neutron absorber 110 may be distributed evenly overabout the rods 34, which reduces the change in local fuel rod power(delta-power) when the GRCA 36 is removed from the core, which in turnimproves operating margins. Further, distributing the absorber material110 evenly over the rods 34 reduces the amount of absorber 110 in eachrod 34, which reduces the amount of heat generated in each rod 34 andresists the risk of bulk boiling in the thimbles 18 under high localpower density conditions. The exact reduction in amount of absorbermaterial 110, as compared with the four Ag—In—Cd absorbers of thecurrent design, is not meant to be limiting upon the invention.

In view of the foregoing, the exemplary gray rod control assembly 36 hasbeen redesigned to address and substantially overcome the aforementioneddisadvantages in the art by including an entirely different absorbermaterial 110 comprising a porous matrix of refractory metal infused witha neutron absorbing metal or metal alloy, a reduced amount of absorber110 in the rods 34 containing the neutron absorber, distribution of theneutron absorber 110 in discrete segments 58 of the rods 34, anddistribution of the neutron absorber 110 evenly among, for example,about twelve rods 34 of the twenty-four rods 33, 34.

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 12. A gray rod control assembly for a nuclear reactor, saidnuclear reactor including a plurality of fuel assemblies each having aplurality of elongated nuclear fuel rods supported in an organized arrayby a number of substantially transverse support grids, and a pluralityof guide thimbles disposed through said support grids and along saidfuel rods, said gray rod control assembly comprising: a spider assemblyhaving a plurality of radially extending flukes; and a plurality of grayrod assemblies coupled to said flukes of said spider assembly, saidspider assembly being structured to move each gray rod assembly withinone of said guide thimbles in order to control the rate of powerproduced by said nuclear reactor, wherein each of said gray rodassemblies comprises: an elongated tubular member having a first end, asecond end, an inner diameter, and a length, a first end plug coupled tothe first end of said elongated tubular member, and being structured tofacilitate insertion of said elongated tubular member into one of saidguide thimbles of said fuel assembly, a second end plug coupled to thesecond end of said elongated tubular member, and being structured to becoupled to one of said radially extending flukes of said spiderassembly, and a neutron absorber comprised of a matrix of refractorymetal fabricated to be porous into which a metal or metal alloy isinfused, wherein said neutron absorber is distributed among a pluralityof said gray rod assemblies.
 13. The gray rod control assembly of claim12, wherein said neutron absorber is disposed as a plurality of segmentswithin most of said elongated tubular member.
 14. The gray rod controlassembly of claim 12, wherein said neutron absorber has a diameter whichis relatively equivalent to the diameter of said elongated tubularmember.
 15. The gray rod control assembly of claim 12, wherein said grayrod control assembly contains twenty-four gray rod assemblies and saidneutron absorber is distributed among twelve gray rod assemblies of saidtwenty-four gray rod assemblies.
 16. The gray rod control assembly ofclaim 12, wherein said neutron absorber is comprised of between about40% to about 80% refractory metal and between about 20% to about 60%metal or metal alloy.
 17. The gray rod control assembly of claim 12,wherein said neutron absorber is comprised of between about 50% to about70% refractory metal and between about 30% to about 50% metal or metalalloy.
 18. The gray rod control assembly of claim 12, wherein saidneutron absorber is comprised of about 65% refractory metal and about35% metal or metal alloy.
 19. The gray rod control assembly of claim 12,wherein said refractory metal is selected from the group consisting ofmolybdenum, tungsten, niobium and zirconium.
 20. The gray rod controlassembly of claim 12, wherein said metal or said metal alloy that isinfused in said refractory metal is silver or silver-indium-cadmium,respectively.
 21. The gray rod control assembly of claim 12, whereinsaid refractory metal is molybdenum and said metal that is infused insaid molybdenum is silver.
 22. The gray rod control assembly of claim 1,wherein said neutron absorber in said plurality of segments disposed insaid elongated tubular member is shaped as cylindrical pellets andspherical pellets.
 23. A nuclear reactor, comprising: a plurality ofelongated nuclear fuel rods each having an extended axial length; anumber of substantially transverse support grids spaced along the axiallength of said fuel rods in order to hold said fuel rods in an organizedarray; a plurality of guide thimbles disposed through said support gridsand along said fuel rods; and a gray rod control assembly including aspider assembly having a plurality of radially extending flukes, and aplurality of gray rods assemblies coupled to said flukes, said gray rodcontrol assembly being structured to move each of said gray rodassemblies within one of said guide thimbles in order to control therate of power produced by said nuclear reactor, wherein each of saidgray rod assemblies comprises: an elongated tubular member having afirst end, a second end, an inner diameter, and a length, a first endplug coupled to the first end of said elongated tubular member, saidfirst end plug being tapered in order to facilitate insertion of saidelongated tubular member into one of said guide thimbles of a fuelassembly, a second end plug coupled at one end to the second end of saidelongated tubular member, and at the other end to one of said radiallyextending flukes of said spider assembly, and a neutron absorbercomprised of a matrix of refractory metal fabricated to be porous intowhich a metal or metal alloy is infused, said neutron absorber disposedas a plurality of segments within most of said elongated tubular member,said neutron absorber having a diameter which is relatively equivalentto the diameter of said elongated tubular member, wherein said neutronabsorber is distributed among a plurality of said gray rod assemblies.24. The nuclear reactor of claim 23, wherein said gray rod controlassembly contains twenty-four gray rod assemblies and said neutronabsorber is distributed among twelve gray rod assemblies of saidtwenty-four gray rod assemblies.
 25. The nuclear reactor of claim 23,wherein said neutron absorber is comprised of about 65% molybdenum thatis infused with about 35% silver.